Histone post-translational modifications (PTMs) are crucial to eukaryotic genome regulation, with a range of reported functions and mechanisms of action. Though often studied individually, it has long been recognized that the modifications function by combinatorial synergy or antagonism. Interplay may involve PTMs on the same histone, within the same nucleosome (containing a histone octamer), or between nucleosomes in higher-order chromatin. Given this, the field must distinguish ever greater complexity, and the context in which it is studied, with brevity and precision. The proteoform was introduced to define individual forms of a protein by sequence and PTMs, followed by the nucleoform to describe the particular gathering of histones within an individual nucleosome. There is now a need to define specific forms of these entities in prose while providing space for experimental nuance. To this end, we introduce a nomenclature that can express discrete PTMs, proteoforms, nucleoforms, or situations where defined PTMs exist in an uncertain context. Though specifically designed for the chromatin field, adaptions of the framework could be used to describe-and thus dissect-how proteoforms are configured in functionally distinct complexes across biology.
